The present invention relates to the field of radar and acoustic technology and, more particularly, to the field of controlling radar and acoustic detection sensitivity in near-field applications, such as personnel intrusion detection.
In radar technology directed toward intrusion control, the target of interest is, or may become, relatively close to the radar antenna and to the ground surface such that the normally used far-field assumptions no longer apply. Control over radar detection sensitivity as a function of near-field range is especially critical in an intrusion detection system where detection sensitivity to the minimum target must be maintained over the required surveillance region without developing areas that are oversensitive to the presence of non-targets, such as birds and small animals, or to the effects of environmental changes. In other types of radar deployed as short ranges, sensitivity control is often important in order to bound the radar dynamic range requirements.
Two methods of accomplishing control over area sensitivity are described in U.S. Pat. No. 3,300,768, entitled "Radiant Energy Type Intrusion Alarm System", to Albin Bystrom, Robert V. Hill, and Herbert A. Williams, assignors to The Boeing Company, Seattle, Wash. Both of these methods employ four antenna elements, deployed as a "multi-static" radar system, in which transmitting and receiving antenna elements occupy separate locations. In a first embodiment, the antennas are placed at the corners of a square so that the surveillance region effectively surrounds the target area. Although all four antennas point approximately toward the center of the protected area, the two transmitting elements are diametrically opposite one another, as are the receiving elements, so that no transmitting antenna element is oriented directly toward a receiving element. The elements used have shaped patterns which are intended to provide even coverage over a circle with the antennas located on its circumference. Range gating is used to restrict sensitivity to the defined area, gating out reflections from large scatterers on the outside.
Although such systems have been and continue to be used, they have developed certain disadvantageous false-alarm and maintenance problems. In order to understand these problems, it must be recognized that the multi-static system is a "forward-scattering" system, in which signals from the site surface as well as from the target are reflected in a forward direction from the transmitting antenna element to a target or the surface and on to the receiving element. Such a system is to be distinguished from a monostatic system in which radiation is backscattered from the target to the same antenna that transmitted the signal. Forward scattering works well on the target, but, unfortunately, the forward-scattered signal from the surface is orders of magnitude larger than that from a human intruder and, moreover, such surface scattering is subject to change with change in the state of the environment (wet, dry, snow, freezing, thawing, etc.). The above-cited reference compensated for environmental state changes by establishing a bridge between the antenna elements that balanced out those large surface-reflected signals. However, erosion and discrepancies in the site grading frequently served to reduce the effectiveness of the bridge. Heavy rain on the antennas was found to make small but uncompensated phase shifts in the large signals between antennas that have caused numerous false alarms. Thermal differences in the cables have caused imbalances in the bridge was well as causing intermittent connections, resulting even in destruction of cable connectors. These systems, with their four long cables and many connectors, and their requirements for delicate phase balancing have become increasingly difficult to maintain.
A number of solutions to overcome the difficulties of the prior art system are proposed herein. A monostatic radar would, for example, solve the problem of the large signals from the surface, since backscattering from smooth surfaces is many orders of magnitude less than specular forward scattering. The plural cables previously needed could be reduced to a single cable between the radar and the antenna feed. If azimuthally omnidirectional antennas are used, circular symmetry of coverage is automatic, and range gating can be used to reduce supersensitivity to interfering changes at the antenna, such as caused by rain, snow, or other precipitation since the antenna is at the center of the region, and is thus self-protecting. Range gating may also be used to restrict coverage to some outer, circular bound. However, the problem of establishing evenness of coverage to a low-profile intruder remains. Sensitivity Time Control (STC) and antenna pattern control through arraying are methods used successfully to attain desired sensitivity control in radars where far-field assumption is applicable; examples are the use of STC voltage gain proportional to the square of the range in antiaircraft fire control radars and of the use of CSC.sup.2 .theta. COS.sup.1/2 .theta. patterns in airborne ground-mapping radars. In near-field radars, single-element sensitivity becomes a much more complicated and rapidly varying function of range, due to phase and amplitude variations of ground-surface reflections, rendering standard monotonic, narrow-bandwidth STC functions unusable. Complex, wide-bank STC functions can be used, but are difficult to implement and suffer a high cost and reliability penalty.
Normal antenna pattern synthesis also cannot be used for near-field cases, both because of the presence of the ground-surface reflections and because the angle from each element or portion of the antenna to the target is different from that of each other. As a result, the composite "pattern" of the antenna is not independent of the range and/or height of the target.